Although the number and variety of mechanical devices and forms of tracheal tubes that have from time to time been devised by the inventive ingenuity of operators is large, the choice practically centres upon one of two forms. The first, and the one most commonly used, is but the original canula of Trousseau, modified by Roger, in that the tracheal portion of the tube is detached from the collar or neck-piece, and moves freely with the movements of the patient; and by Obré, by the important device of an inner tube to prevent clogging of the outer or original tube by mucus. Starting upon this essential basis, the instrument-maker has perfected the instrument of to-day. It is a silver tube, double throughout, the inner tube projecting at the lower or tracheal end beyond the outer--an important point, as it prevents any possible permanent occlusion by mucus or blood-crusts, membranes, and the like at this point, removal of the inner tube at once clearing the end of the outer one. The curve of both tubes should correspond to the arc of a quadrant, and the outer is fastened to a transverse collar or shield by means of two small projections or pins upon its sides which lie under small wire bridges upon the shield after it has passed through an opening in the {151} transverse neck-collar large enough to permit of its free movement during the respiratory movements of the trachea, as well as during the forcible action caused by cough. The ends of this collar or shield curve slightly backward to correspond with the curve of the neck, and are perforated by, preferably, large oval openings, instead of the usual small, inconvenient slit, through which the tapes are passed which hold the tube in position by encircling the neck. To this same shield is fastened, by means of a small turn-screw or a revolving collar, the end of the inner tube, which is thus prevented from being forced out of the outer tube by coughing or any motion of the patient. Upon the upper or convex surface of the outer tube a small ovoid opening is usually made for the purpose of permitting the expiratory current to pass upward (the inner tube being removed) into the larynx and render phonation possible; also, the free opening of the outer tube being closed, to allow of respiration being carried on through the larynx and natural passages--often an important matter, as the case progresses toward recovery, in instances where the operation of tracheotomy has been performed on account of laryngeal obstruction.